1. Field of the Invention
The present invention relates to a method of compensating for a measurement error in an apparatus which measures the insulation resistance of an electric power transmission line, a grounding line for lightning protection or the like.
2. Description of the Related Art
There has been so far known in electric power transmission facilities such a measurement apparatus which monitors the insulation resistance between an electric line and the ground and detects the insulation deterioration of the line as fast as possible, thus preventing beforehand any trouble from occurring for the stable transmission of electric power. An example of such a measurement apparatus is given by an electric circuit in FIG. 1, which measures an insulation resistance R.sub.0 between an electric line and the ground in the event where the electric power of a voltage stepped down by a transformer T is supplied or transmitted through electric lines 1 and 2 on of which is connected to the earth E through a grounding conductor L.sub.E.
More specifically, the aforementioned circuit of FIG. 1 is arranged so that a transformer OT, which is connected to a low frequency signal oscillator OSC generating a measuring signal of a low frequency different from a commercial power source frequency, is inserted in the grounding line L.sub.E of the power receiving transformer T to apply a measuring low frequency voltage to the electric lines 1 and 2; a current transformer ZCT having the grounding line L.sub.E passed therethrough detects a leakage current of the aforementioned measuring low frequency signal that is fed back to the grounding conductor through the insulation resistance R.sub.0 and an earth stray capacity C.sub.0 existing between the electric lines and the earth; an amplifier AMP connected to the current transformer ZCT amplifies the detected leakage current; a filter FIL connected to the amplifier AMP extracts only a frequency f.sub.1 component from the amplified signal; and a multiplier MULT synchronously detects the extracted component with use of, for example, an output signal of the oscillator OSC to detect an effective component (OUT.sub.1) (that is, a component in phase with the applied low-frequency voltage) of the leakage current and to thereby measure the insulation resistance of the electric lines.
Explanation will next be made as to the measuring theory.
Assuming now that the measuring signal voltage applied to the grounding line L.sub.E is of a sine wave V sin .omega..sub.1 t (.omega..sub.1 =2.pi.f.sub.1), then a leakage current I of a frequency f.sub.1 fed back to the grounding line L.sub.E through an earth point E is expressed by the following equation. EQU I=(V/R.sub.0).multidot.sin .omega..sub.1 t+.omega..sub.1 C.sub.0 V cos .omega..sub.1 t (1)
The leakage current I extracted by the current transformer ZCT and passed through the amplifier AMP and the filter FIL is synchronously detected by the multiplier MULT with the signal of the oscillator OSC in phase with the low-frequency signal applied to the electric lines to extract its effective component, i.e., the first term in the right side of the above equation (1). The effective component, which is inversely proportional to the insulation resistance R.sub.0, can be used to find the insulation resistance of the electric lines. With such a prior art method of detecting at the zero-phase current transformer ZCT the leakage current fed back to the grounding line and extracting and outputting at the filter FIL the component having a frequency of f.sub.1 from the leakage current, however, when the leakage current component of the frequency f.sub.1 is shifted in phase through passage of a system comprising the zero-phase current transformer ZCT, the amplifier AMP and the filter FIL, it becomes impossible to calculate the value of the insulation resistance accurately. To avoid this, it has been conventional to use a phase shifter which adjusts with respect to phase one or both of the signal sent to the multiplier MULT, i.e., the signal sent from the low frequency oscillator OSC and the leakage current passed through the extracting filter FIL from the current transformer ZCT to thereby set or correct a phase difference between the both signals to be zero.
However, the prior art method has been defective in that the phase characteristics of the current transformer ZCT, filter FIL, phase shifter and so on vary with temperature variations, the deterioration of characteristics of used parts with age and so on, which results in that a phase error from the initial adjustment value takes place, thus making it impossible to provide a correct measurement result. To cope with the defect, there has been so far employed such a high quality of zero-phase current transformer, filter and the like that are very small in their characteristic variations to thereby minimize the influence due to the phase error. Even so, it has been impossible to completely eliminate the influence.
More in detail, if the leakage current component I of the frequency f.sub.1 shown in the equation (1) is assumed to have a phase shift .theta. when passed through the system of the zero-phase current transformer ZCT, amplifier AMP and filter FIL, then the filter FIL produces such an output I.sub.1 as follows. EQU I.sub.1 =(V/R.sub.0) sin (.omega..sub.1 t+.theta.)+.omega..sub.1 C.sub.0 V cos (.omega..sub.1 t+.theta.) (2)
And the output I.sub.1 is applied to a first input terminal of the multiplier MULT.
Assuming a voltage applied to a second input terminal of the synchronous detector is, for example, a.sub.0 sin (.omega..sub.1 t+.theta..sub.1) of a constant amplitude, then an output or an effective component D of the synchronous detector is expressed as follows. ##EQU1## where -- means to eliminate components of D above angular frequency .omega..sub.1.
Hence, an output D.sub.0 when .theta.=.theta..sub.1 is given as follows. EQU D.sub.0 =Va.sub.0 /2R.sub.0 ( 5)
Since V and a.sub.0 are constant, the output D.sub.0 can be measured as a value inversely proportional to the insulation resistance R.sub.0. Accordingly, an error E for the effective component D with respect to the output D.sub.0 when the phase shift (.theta.-.theta..sub.1) is not zero becomes: ##EQU2## For example, when .theta.-.theta..sub.1 =1 degree, R.sub.0 =20 K.OMEGA. and C.sub.0 =5 .mu.F, f.sub.1 =25 Hz and .omega..sub.1 C.sub.0 R.sub.0 .apprxeq.15.7. This yields 27.4% of an error .epsilon. with a remarkably large measurement error.
It is an object of the present invention to provide a phase correcting method in an insulation resistance measuring apparatus, which eliminates the above defects in the prior art insulation resistance measuring method, and which can automatically correct a phase shift in a measurement signal inexpensively without the need for any expensive parts and can produce always a correct measurement result.